1. Field of the Invention
The present invention relates to a hollow molded foam article in which a plurality of polypropylene resins are used as the base resin, and to a process for the production of a hollow molded polypropylene resin foam, and more particularly relates to the use of a plurality of polypropylene resins having specific melt tensions and melt flow rates.
2. Description of the Related Art
Blow molding technology has been utilized in the past to obtain blow-molded foams having a foam layer. Various methods have been proposed in the past for obtaining blow-molded foams, but the most common is a method in which a foaming agent and a base resin are melt-kneaded in an extruder, this mixture is extruded from a die, the cylindrical foam thus formed is placed in a metal mold, and blow molding is performed by blowing a pressurized gas into the interior of this cylindrical foam.
Blow-molded foams produced by this method can be used in applications that require thermal insulation, sound-proofing, flexibility, and other such properties, specific examples of which include containers, ducts, thermal insulation panels, and automotive parts. The expansion ratio used in these applications is 3 to 30 times. For instance, there is a known blow-molded foam in which polystyrene or polyethylene is used for the base resin and the expansion ratio of the foam layer is 4 to 12 times (see Japanese Patent Publication H3-59819, for example).
When the heat resistance of the molded article is taken into account, it is preferable to use a polypropylene resin as the base resin, and there have been numerous proposals for foams made from polypropylene resins because of the good heat resistance and rigidity of these resins. However, nearly all blow-molded foams made using a general purpose polypropylene resin as the base resin had a low expansion ratio in which the apparent density was over 0.3 g/cm2 when foamed with a chemical foaming agent.
When a general purpose polypropylene resin is used as the base resin and is foamed with a physical foaming agent, foaming occurs when the foaming agent kneaded into the base resin expands, but with a polypropylene resin, changes in the temperature of the molten resin near the foaming temperature result in large changes in the melt tension and viscosity of the molten resin, and if the temperature of this resin is even just a little too high, the viscosity and melt tension will decrease to the point that the foaming agent cannot be held in the resin, the result being that the foaming agent escapes from the polypropylene resin during extrusion and foaming, the cells become continuous or rupture, and a good cylindrical foam cannot be obtained. In a worst case scenario, foaming itself is impossible. Conversely, if the melting temperature of the polypropylene resin is lowered in order to keep the viscosity and melt tension of the polypropylene resin high, the polypropylene resin undergoes crystallization, which prevents adequate and uniform foaming from being achieved.
There is only a narrow range of temperatures suited to the foaming of polypropylene resins, and it has been difficult to obtain a blow-molded foam with thick walls and a high expansion ratio using a general purpose polypropylene resin, but the present applicant has proposed a blow-molded foam with thick walls and a high expansion ratio, and a process for the production of this foam, that overcome this difficulty, in which a polypropylene resin having a specific melt tension and a specific melt flow rate is used as the base resin (for example, International Laid-Open Patent Application WO/99/28111).
Using a specific polypropylene resin having a specific melt tension and melt flow rate as above does produce a thick-walled foam with a good appearance, having a high expansion ratio, and having excellent heat resistance, thermal insulation properties, and so forth. Nevertheless, the above-mentioned specific polypropylene resins are expensive, which drives up the cost of the molded foam finished product thus obtained, so this is extremely disadvantageous from a cost standpoint except in special applications. Also, a large amount of flash is produced in the production of blow-molded foams, and the use of recycled material also poses problems.
Therefore, in light of the above situation, it is an object of the present invention to provide a thick-walled hollow molded foam with a good appearance and a high expansion ratio by using a plurality of polypropylene resins having specific melt tensions and melt flow rates, rather than using just one specific polypropylene resin having a specific melt tension and melt flow rate, as well as a process for the production of this foam.